The present invention relates to an interference canceller used for code division multiple access (CDMA) systems.
CDMA systems which are based on direct-sequence spreading, are usually capable of greatly expanding the subscriber's capacity, and are thus attracting attention as the multiplex access system for mobile communication systems. In the CDMA system, each user's signal is spread with a unique spreading code in a wide frequency bandwidth and sent to a transmission channel. In the receiver side, a desired user's signal is detected from the code multiplexed received signal through despreading process. In this system, a correlation among user's spreading codes, would cause an interference and degrade the receiver performance. To remove the interference, a practical interference canceller, which performs despreading using an adaptively determined orthogonal coefficients, has been proposed. A literature concerning this interference canceller is Yoshida, Ushirokawa, Yanagi, and Furuya, "DS/CDMA Adaptive Interference Canceller on Differential Detection for Fast Fading Channel",IEICE Transactions on Communications, Vol. J77-B11, No. 11, November 1994 (Japanese Patent Application Heisei 5-169092, and Japanese Patent Publication Heisei 6-307275).
FIG. 5 shows an example of the prior art CDMA interference canceller shown in the literature.
A code-orthogonalizing filter 201 despreads a code-multiplexed received signal using an orthogonal coefficient, which are independent of channel variations, and it thus detects a desired user's signal while suppressing interferences from other users. The orthogonalizing filter 201 is constructed as a transversal filter, and its tap interval is suitably designed to be a fractionally chip-spaced from the consideration of asynchronous interferences. This despreading is different from the despreading using the conventional matched filter in that the orthogonalizing filter 201 uses an adaptively determined orthogonal coefficients as the filter coefficients in lieu of using a spreading code used in the transmitter. A carrier tracking circuit 202 effects carrier phase synchronization of the despread desired user's signal. A symbol decision unit 203 decides the most possible transmitted symbol from the output of the carrier tracking circuit 202. By the term "symbol" is meant the transmitted modulation signal, and in the case of binary phase modulation, for instance, it is "1" or "-1". In this case, the symbol decision unit 203 outputs "1" when the carrier tracking circuit output (analog value) is positive and "-1" when the carrier tracking circuit output is negative. Where multi-level amplitude and phase modulation is used, the decision region becomes two-dimensional (a complex plane). An adder 204 extracts a symbol decision error signal. A tap coefficient updating means 205 updates the orthogonal coefficients recursively on the basis of the symbol decision error signal outputted from the adder 204.
The tap coefficient updating means 205 receives, as its inputs, the input to the orthogonalizing filter 201, a recovery carrier outputted from the carrier tracking circuit 202 and the symbol decision error signal, and determines the orthogonal coefficients such that the mean power of the symbol decision error signal is minimum. This control is called minimum mean square error (MMSE) control. A least mean square (LMS) algorithm is well known as a method for easily realizing the MMSE control. An example of operation of the tap coefficients updating means 205 when the LSM algorithm is used is as follows.
The tap coefficients vector c(i+1) is expressed as: EQU c(i+1)=c(i)+Re.mu.r*(i)x(i)e(i)!
and the symbol decision error signal e(i) is expressed as: EQU e(i)=d(i)-c.sup.T (i)r(i)x*(i)
where r(i) is an input signal vector to the orthogonalizing filter 201, x(i) is the recovery carrier outputted from the carrier tracking circuit 202, d(i) is a decision symbol outputted from the symbol decision unit 203, * is the complex conjugate, .sup.T is the matrix transpose, Re.sup.. ! is the process for taking the real part, and .mu. is the step size.
As shown, in the prior art CDMA interference canceller, the tap coefficients are recursively updated according to the symbol decision error signal, and the interference cancellation can be performed adaptively and easily according to changes in the spreading timing and power of interference signal. In the initial converging of the tap coefficients, however, the decision symbol usually lacks reliability, and training signal is needed in place of the decision symbol. As the training signal, a symbol pattern which is known to the receiver side is transmitted from the transmitter side.
In the prior art CDMA interference canceller as described above, in which the training signal is required at the time of the initial convergence, the tap number of the orthogonalizing filter 201 is set to a few times the number of the spreading code length. Therefore, for the convergence, the training signal of several ten times the tap number is necessary. The data transmission efficiency is therefore extremely reduced particularly for the burst data transmission. In addition, it requires a process including operations that the interference canceller detects the establishment of the convergence and transmits the detection signal to the transmitter and that the transmitter switches training signal over to the information data signal. A further problem is that when the orthogonalizing filter 201 gets into the loss of the synchronization, detection of the synchronization loss and re-sending of training signal for re-convergence are necessary, thus complicating the process.
For the initial convergence of the tap coefficients, a blind operation requiring no training signal is desirable. As a literature concerning blind CDMA adaptive interference cancellers is M. L. Honig, U. Madhow, S. Verdu, "Blind Adaptive Interference Suppression for Near-Far Resistant CDMA", Globecom, "94, pp. 379-384. According to this literature, a constraint minimum output power algorithm is used as the tap coefficients updating means for an adaptive filter for interference suppression. In this process, however, the desired signal power versus interference power (SIR) ratio after the convergence is not so satisfactory.